run the single test file specified in the args param

This change makes it possible to run the tests in a single file by specifying its class name in the *args* URL parameter, i.e. to run just the *TestInterIsolateMutex* tests, you would specify `args=com/sun/midp/util/isolate/TestInterIsolateMutex` in the URL params.
I mostly did it because it seemed straightforward, easy to read and understand (although not quite as easy as `Arrays.asList(Testlets.list).contains(…)`, which unfortunately isn't available in the J2ME class library).

If I didn't create the vector, I would have to loop over the *Testlets.list* array to see if it contains the test, which is also relatively simple, and faster, on average (although performance isn't an factor here).

I was also thinking about supporting multiple tests by reading all the args instead of just the first one, at which point a vector would make it simpler to find each test; but I haven't done that yet.

I could also have modified Testlets.java and its Makefile recipe to make *Testlets.list* be a vector, but that would have been complicated.